MECHANICAL EVENT SIMULATION AIDS IN OUT-OF-COURT SETTLEMENT OF FAULTY PRODUCT LAWSUIT

		This close-up shows the seat with its supporting pedestal. (Photograph Courtesy of Herman M. Giesen, P.E.)
		By destructively examining a sample seat pedestal and the design drawings, Giesen discovered design flaws that caused a stress concentration in the scissor mechanism's axle sufficient to cause failure over time. In the photo of the assembled seat pedestal (upper right), the axle in question is located in the upper right corner of the mechanism. Notice in the close-up of the axle after destructive examination (lower left) that 1) there is no fillet at the axle root, and 2) the hole through which the axle passes is tapered. Giesen concluded that these design flaws were a likely cause of fatigue and failure given chronic cyclical and vibrational stress. (Photographs Courtesy of Herman M. Giesen, P.E.)

It started as an ordinary day at the TU Electric Oak Hill strip mine site near Tatum, Texas. What wasn't ordinary was the unexpected collapse of the excavating machine's seat pedestal, after which the excavator operator reported that the accident had injured his lower back. In the subsequent faulty product lawsuit brought forth by the strip mine's insurance company, the Henderson, Texas law firm of Wellborn, Houston, Adkinson, Mann, Sadler & Hill, L.L.P. was presented with the challenge of proving that the mechanical failure of the seat caused the operator's lower back injury. 

The law firm contacted Herman M. Giesen, P.E., a Dallas, Texas engineering consultant, to research the cause of the failure by reviewing design drawings and destructively examining a sample excavator seat pedestal. The lawsuit would also require that Giesen find a way to estimate the impact suffered by the operator's lower spinal column when his seat collapsed. Giesen used Mechanical Event Simulation software from Pittsburgh-based ALGOR, Inc. to provide an accurate engineering simulation of the impact on the operator's lower spinal column. The simulation showed the resultant impact forces, stresses and oscillating movement, which (among other factors) compelled the defendant to settle out-of-court, avoiding a lengthy, costly trial by jury.

Researching the Cause of the Failure
Giesen's research into the cause of the failure focused on the seat's pedestal assembly, based on a witness' report that the seat's failure had apparently occurred in the area of the seat torsion assembly axle. The seat of the excavator rests on a pedestal assembly that includes an adjustable torsion spring scissors mechanism. This mechanism enables the operator to adjust the seat to a height that enables him to comfortably reach two foot pedals as well as a hand manipulator.

Unfortunately, Giesen was unable to work with the seat pedestal involved in the accident, which had been discarded at the site. However, Giesen was able to obtain a sample seat pedestal for destructive examination and the seat pedestal design drawings from the manufacturer through the legal discovery process. 

By simply reviewing the design drawings, Giesen detected two important facts about the design: 1) the bearings which support the axles overhang the axle roots, and 2) there are no fillets at the roots of the axles. The effect of the first fact is to create a significantly overhung load, which heavily stresses the root under a bending load. This stress concentration at the root is further and heavily amplified by the second fact - the absence of root fillets. 

"Failure to provide axle root fillets was a design flaw and was the root cause of the failure and the operator's injury. Had appropriate fillets been provided, the event most probably would not have occurred," said Giesen. "Given these design flaws, chronic cyclical and vibrational stress are likely to cause fatigue cracks to develop, propagate and cause failure."

As he moved forward with the destructive disassembly and examination of the sample seat pedestal to confirm that there were no fillets at the axle roots as manufactured, he also discovered that the axle hole called for in the flat bars that supports the axle was tapered and ragged, probably because the hole was punched in manufacturing. "The tapered axle hole would have allowed for an axle root fillet radius of approximately 0.02 inch," explains Giesen. "A fillet of that size would have significantly reduced the stress concentration factor and hence the likelihood of the failure. Alternatively, a non-tapered hole would have better supported the axle root as machined. Either way, it was unambiguously clear that no relief had been specified in the machining of the axle root." 

Once Giesen had established that design and manufacturing flaws were causal factors in the failure of the seat pedestal, the lawyer posed a question that would be important if the case went to trial: How much force was actually involved in the impact.

"Simple question," said Giesen, "but tough to answer." How to find the impact force was a problem that would lead Giesen to utilize the unique capabilities of ALGOR's Mechanical Event Simulation software. 

Simplifying the Tough-to-Answer Question
"Since I am not an expert in mechanical analysis of this sort, I sought the council of mechanical analysis experts I respect," said Giesen. "From them I learned that quantifying an impact force defies traditional methods provided by handbooks and calculations in practical terms - you just can't do it." 

Then Giesen learned about ALGOR's Accupak/VE Mechanical Event Simulation in a trade publication. ALGOR's finite element analysis-based Accupak/VE Mechanical Event Simulation software with linear and nonlinear material models, realistically simulates motion and flexing in mechanical events, eliminates the need to input forces and computes and shows resulting stresses on the computer model at each instant in time. It can even show dynamic effects such as vibration in real time on a model as the event unfolds. Because events are defined by modeling the actual physical scenario, rather than applying forces to an artificially constrained model, Accupak/VE Mechanical Event Simulation software can even calculate the forces at work on a model at each moment. 

Since Giesen has extensive mechanical and electro-mechanical design and system engineering experience but very little hands-on experience with finite element analysis (FEA) and Mechanical Event Simulation, he arranged for an Individualized Education Seminar at ALGOR's Education Center, located at their Pittsburgh, Pennsylvania World Headquarters. During this three-day, personalized session with an application engineer experienced in FEA and Mechanical Event Simulation, Giesen developed the impact model. "The training I received was of critical value," said Giesen. "Not only was I able to complete the required analyses in just three days, it was also a wonderful educational experience."

Developing a Model to Determine the Impact Force of the Seat's Sudden Collapse
The model Giesen developed was modeled in Superdraw III, ALGOR's single user interface and precision finite element model-building tool, based on manufacturer design drawings and information about the operator provided by the lawyer. Where numerical values were not available, they were estimated using reasoned and conservative engineering judgment and varied over several iterations. 

The seat assembly was modeled in an upright position using 3-D beam, plate/shell and solid brick elements. The beam and plate/shell elements were used to represent the steel base of the seat and were defined using the material properties of steel from ALGOR's Material Library Manager. The seat cushions were modeled using solid brick elements and were defined using a custom material. The density of custom material was defined such that the seat assembly would weigh a total of 120 pounds. Giesen researched common material property values of polyurethane foams at the University of Pittsburgh engineering library to establish a reasonable range of Young's moduli for the cushion material properties. However, the Young's modulus of the seat cushioning was one of the variables that would be altered over a series of iterations. 

Although the seat actually dropped 3-5 inches, Giesen's model assumed a 2-inch dead drop onto a stiff floor. The contact between the seat and the floor was modeled using ALGOR's proprietary contact elements, which enable engineers to model how parts of a mechanism behave when they come into contact. By fixing each 2.5 inch contact element at the bottom and specifying .5 inch as the length at which the elements would become rigid, contact with the floor was simulated without actually modeling the floor. 

An assembly representing the operator in an upright posture - perched on the seat to reach the excavator's controls - was then added to the seat assembly. The arms, legs, body and head were modeled using solid brick elements with a density such that the weight totaled 288 pounds. The weight of the operator (360 pounds) was discounted conservatively by 20% to account for the fact that he was perched on the seat with the foot pedals and hand manipulator supporting some of his weight. Boundary conditions fixed the hands where they would have grasped the hand manipulator and feet where the heels would be resting on the floor. Beam and contact elements were used to represent the knee and shoulder joints. The spine consisted of truss elements. Although the human spine naturally has a curved shape, the spine was modeled perfectly straight to simplify the issue of impact force. If the backbone had been curved, it would have deflected in the analysis thus absorbing much of the force, rather than calculating a total impact force as was intended. The Young's moduli for the body parts was based on biomechanical information resources and were varied over a series of iterations. 

The complete model was subjected to a standard gravity loading for a duration of 1 second analysis with 100 time steps per second. During processing, ALGOR's built-in visualization capabilities were activated so Giesen could watch the event unfold as it was processed. ALGOR enables WYSIWYG (What-You-See-Is-What-You-Get) visualization by showing the movement of the mechanism and stresses as they occur over time. Thus, Giesen could vary the stiffnesses of the seat cushions and body based on the behavior of the model. 

Giesen evaluated the results at the moment of impact in g's, for force amplification factor, which is a factor of how much a subject's body weighs at the time of impact. Depending on the input variable, some of Giesen's models yielded an impact force of as much as 5 and 6 g's. However, Giesen's final, optimized model, which represented a conservative maximum result, yielded 2.24 g's. According to that result, the operator's weight at the moment of impact was effectly 645.12 pounds - a considerable weight for the human spine to bear.

However, Giesen was more impressed by the dynamic effect that could be seen taking place in the spine after impact. "It was remarkable to see the reverberation in very high mechanical frequency rippling up and down the backbone," said Giesen "The strongest beneficial result of ALGOR's Accupak/VE Mechanical Event Simulation software was being able to see the force operating at points on the body at issue in real time. The coloration representing results enables quantitative interpretation of the impact event in real time." 

Giesen's report of his destructive examination and Mechanical Event Simulation results were one of many considerations in the subsequent settlement of the lawsuit. Although Giesen never got the opportunity to impress a jury with the results of his simulation, he was impressed with the unique capabilities of Mechanical Event Simulation. "I don't know of any other practical method for quantifying impact force," said Giesen. "ALGOR's Accupak/VE Mechanical Event Simulation software turned out to be the best, right and perhaps only way to answer the question of the impact force's magnitude and illustrate those results."